I was recently taught the concept of a shinui to be "something done in a way that is unusual compared to the norm in one's area of residence", with respect to the permissibility of actions done on Shabbat and Yom Tov.

My question is "what constitutes the norm"? What if 51% of people do something one way and 49% of people do it another way? Furthermore, what constitutes one's "area of residence" in this matter? Is one's "area of residence" confined to one's home, neighborhood, city, country, state, continent, planet?

Is the question phrased clearly enough? It hasn't seen much action and, if it needs improvement, I'm happy to provide it.
– LeeJun 14 '13 at 19:55

1

[continued] shinui, which you would be asur rabbinically, is if lets say you write with your feet. Now, the paper on pen is actually done in a weird way. That is, the ACTION is being done in a weird way, as opposed to standing on your head where the action is still being done properly. So, a better way to define shinui is "the actuall action is done in an unusual way."
– user2995Jul 11 '13 at 17:45

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T0 clarify, Shinui's definition isn't exactly as the way it was defined for you. For example, when dealing with melacha on Shabbos, if someone decides to write on Shabbos by using a shinui, lets say he writes while standing on his head, your definition would constitute him doing a melacha with a shinui. However, this isn't true. His paper on pen isn't construed at all, it's the same result! and he would therefore be chayiv medoraisah, as opposed to a rabbinical issur which you would receive if you actually did the melacha with a shinui. Using the case of writing, a proper case of [continued]
– user2995Jul 11 '13 at 17:45

If a Melocho is performed in what is Halachically recognized as an
irregular mode, it may be considered a "Shinui" (שינוי). ....a Melocho
performed in an awkward or backhanded manner does not conform with the
character of the Melochos in the Mishkan.

Anything that is not usually done in this fashion, or if it is difficult to train one's hand to do it properly, that is a
שינוי

In that footnote, he cites the Chiddushei HaRan to Shabbos 103a (sv אמר אביי, at the end)1, as proof to this idea. When someone writes on Shabbos, he is only liable on a Biblical level when he writes with whichever hand he usually writes with; but someone who carries from one domain to another is liable whether he used his right hand or his left -- it's normal to carry with either hand, and doesn't take much training or getting used to in order to do it in an efficient manner. Writing, on the other hand, is unusual to find righties writing with their left hands, and it's pretty difficult to teach yourself how to be ambidextrous (believe me, I've tried ;).

1Note: You won't find this in the regular blue, Mossad Harav Kook Ran on Shabbos. Many thanks to Fred for providing the link in comments. :)

@Fred I'd have to check, but I'm pretty sure that that's the exact quote that Rabbi Ribiat brought. Thanks for clearing my confusion :) ....do you know where I could find a copy, so I could see the original, "inside"?
– ShokhetDec 17 '14 at 18:04